Recently, large-scale display devices, car navigation systems, laptop-type personal computers, and the like that are used as OA equipment and AV devices are increasingly required to be light in weight, thin, and power-saving. Therefore, as we move into multimedia society, light-weight, thin and power-saving display elements such as liquid crystal display elements are regarded as key devices, and a lot of efforts are being made on the research and development of such elements in various fields of OA equipment and AV devices.
Conventionally liquid crystal display elements containing thin film transistors (TFT) as switching elements are well-known. Many of the liquid crystal display elements configured as above adopt a TN (Twisted Nematic) display mode in which the nematic liquid crystal molecules are twisted about 90.degree. near the electrode substrates or an STN (Super Twisted Nematic) display mode in which those nematic liquid crystal molecules are twisted 180.degree. or more near the electrode substrates.
The aperture ratio greatly affects the display performance of the liquid crystal display elements, such as the light transmittance, power consumption, and viewing angle characteristics.
For example, the use of a transparent-type liquid crystal display element having an active matrix substrate, which will be described in the followings, is suggested for an improvement of the aperture ratio.
Gate signal lines and source signal lines are disposed on the insulating substrate of the active matrix substrate so as to cross each other. A switching element is provided near the crossing points of the gate signal lines and the source signal lines. The switching element is connected at its gate electrode to the gate signal line, and at its source electrode to the source signal line. Moreover, an interlayer insulating film and a pixel electrode are stacked in this sequence to cover the switching element, the gate signal line and the source signal line. The interlayer insulating film is made of a photosensitive organic thin film exhibiting high transmittance in the visual radiation range. The pixel electrode is made of a transparent conductive film. The pixel electrode is connected to the drain electrode of the switching element.
The configuration, in which the interlayer insulating film is formed to separate the gate and source signal lines from the pixel electrode, allows the pixel electrode to be formed so as to overlap the gate and source signal lines, and thereby increases the aperture ratio of the transparent-type liquid crystal display element.
However, those various films composing the active matrix substrate do not exhibit satisfactory adhesion.
For instance, as for the above liquid crystal display element, the cross-sectional structure of the active matrix substrate in its pixel section is insulating substrate/inorganic insulating film/organic insulating film/transparent conductive film/orientation film. Again as for the above the liquid crystal display element, the cross-sectional structure of the active matrix substrate in its sealing section is insulating substrate/inorganic insulating film/organic insulating film/sealing member.
The active matrix substrate configured as above and the liquid crystal display element incorporating this active matrix substrate have a low adhesion strength of about 5 kg/cm.sup.2 to 8 kg/cm.sup.2. For example, after the active matrix substrate and the liquid crystal display element were preserved for 18 hours in accordance with pressure cooker test (PCT), the films was peeling between the organic insulating film and the transparent conductive film and/or between the organic insulating film and the inorganic insulating film in the sealing section and the pixel section. PCT is a preservation test at a temperature of 121.degree. C., pressure of 2 atm, and humidity of 99%.
Moreover, the films peel also when the transparent conductive film deposited on the organic insulating film is formed into a pattern to form a pixel electrode, since the resin composing the organic insulating film swells with the resist removing liquid (e.g., DMSO).
The peeling, regardless of the kind of the film that peels, shortens the lifetime of and thus decreases the reliability of the display elements such as the liquid crystal display element.
For these reasons, there is a strong demand for a substrate for use in display elements, that exhibits excellent adhesion between the films of which the substrate is composed.